In forming a radiation collector, it is desirable to increase as much as possible the absorption of the collector while decreasing as much as possible the amount of heat lost from the collector. It has long been known that an excellent radiation absorber is a body having a hollow spherical cavity with a black wall and an opening to the cavity for receiving radiation which is extremely minute and minimizes as much as possible emission through the opening by radiation. While a hollow spherical cavity of this type is an ideal radiation absorber, it is very difficult to have it absorb any great amount of energy due to the minute opening leading to the interior of the cavity. Furthermore, it is difficult to effectively and continuously remove heat therefrom.
Where the radiation collector is a solar collector and it is desired to remove as much heat as possible from the collector, it is practically necessary to have some kind of heat exchange fluid which is circulated in contact with the walls of the radiation collector to continuously remove heat from the radiation collector as it is absorbed from the sun. This requires that the collector not only have as ideal characteristics as possible for effective absorption of energy but that it also has a form and shape which makes it possible to readily circulate a heat exchange fluid in heat exchange relation therewith. Furthermore, if any great amount of solar energy is to be absorbed, the construction must be one which makes it possible to provide a very large number of radiation absorbing cavities in a relatively small space in a structure which can be readily produced and which can be utilized under the available conditions commonly present for absorption of solar energy.